1. Field of the Invention
The present invention relates to a demodulating mechanism of a communication system. More particularly, the present invention relates to a demodulation mechanism utilizing two-level differential amplitude-shift-keying and a communication system using the same.
2. Description of the Related Art
In this modern world, communication techniques and communication systems play very important roles in daily life. Popular communicating systems include mobile phones and wired/wireless networks. In recent years, in the wire/wireless network and digital terrestrial broadcasting applications, orthogonal frequency division multiplexing (OFDM) transmission techniques based on multi-carrier modulation are useful in overcoming multi-path reflection effects. The multi-path reflection effects originate when, in digital communication or broadcasting application, information signals in the form of electromagnetic waves are transmitted from a transmitter through a physical channel (for example, air) to a receiver. Due to the non-ideal properties of the physical channel, multi-path reflection and attenuation occur so that the input signals are distorted.
According to the modulation technique applied, the OFDM system can be categorized as coherent OFDM systems and non-coherent OFDM systems. For coherent modulation at the receiver of the coherent OFDM system, the transmitter transmits pilot signals and information signals. The receiver performs channel estimation and equalization operation based on pilot signals and compensates for distortion due to channel effects. Popular coherent OFDM systems applying quadrature amplitude modulation (QAM) include: digital video broadcasting-terrestrial (DVB-T), asymmetric digital subscriber line (ADSL), very-high-speed digital subscriber line (VDSL) and wireless local area network (WLAN) 802.11a/g/n.
In the non-coherent OFDM system, the transmitter utilizes differential modulation to transmit the difference (the relative magnitude instead of the absolute magnitude) between neighboring transmitted signals. The receiver uses reverse differential demodulation techniques to recover the transmitted signal. The compensation for channel effect is already existed within the differential demodulation process. Non-coherent OFDM systems applying differential quadrature-phase-shift-keying (D-QPSK) include: digital audio broadcasting (DAB), home plug and integrated services digital broadcasting-terrestrial (ISDB-T).
The coherent OFDM system and the coherent OFDM system have drawbacks and merits. The non-coherent OFDM system has two major advantages: firstly, no frequency bandwidth sacrificed in transmitting the pilot signal at the transmitter; and, secondly, a simple receiver because no channel estimation and equalization are required. However, the non-coherent OFDM system also has its drawbacks. The non-coherent OFDM system has a worse theoretical demodulating performance than the coherent OFDM system.
The aforementioned non-coherent OFDM system uses the D-QPSK modulation technique. Hence, the corresponding demodulation just requires simple multiplication operations. Furthermore, using the differential phase-shift-keying (D-PSK) in the same bandwidth for increasing the number of phase bits increases the efficiency of the frequency spectrum in the non-coherent OFDM system. Although this method can still get aforementioned advantages, higher signal-to-noise ratio (SNR) will lead to significant drops in the operation efficiency of the system. Therefore, the differential amplitude—phase-keying (D-APSK) modulation technique has been developed for the non-coherent OFDM system to increase its spectrum efficiency. Nevertheless, the D-APSK modulation technique combines the differential phase-shift-keying (D-PSK) technique and the differential amplitude-shift-keying (D-ASK) technique together.
In the following the D-ASK signal modulation and related conventional techniques are described. The first conventional technique is disclosed in the U.S. patent application publication No. 2004/00366471, titled “Differential amplitude detection diversity receiver”. The first conventional technique uses a divider to serve as a diverse antenna receiver for demodulating the received D-ASK signal.
In the first conventional demodulation technique, three devices, an absolute value unit, a delay unit and a divider unit, are used to demodulate the differential amplitude signal and the result is transmitted to an amplitude selection unit for evaluating the magnitude of the amplitude. Since the method of determining the differential amplitude requires a divider, the implementation is highly complicated.
The second conventional demodulation technique is disclosed in U.S. Pat. No. 5818875, titled “Modulation and demodulation method, modulator and demodulator”. The second conventional technique uses a logarithmic-to-linear conversion to demodulate the D-ASK signals. In the second conventional technique, the amplitude of the input signals is transformed to logarithmic value so that the division operations with linear values are implemented by subtraction operations with logarithmic values. After obtaining a difference value in the logarithmic domain, it is converted back to a linear value in the linear domain so that a differential quantity is produced. This replacement method for division is more aptly applied to an analog circuit design. If implemented in digital, a look-up table is needed in the logarithmic/linear area conversion. Ultimately, the sophistication in hardware implementation is a problem.
The third conventional demodulation technique is disclosed in U.S. Pat. No. 6,148,007 titled “Method for data transmission in a digital audio broadcasting system”. The D-ASK signals are demodulated according to the following schemes:|R[n]|<|R[n−1]|*((1+a/b)/2)→u[n]=1,|R[n]|>|R[n−1]|*((1+b/a)/2)→u[n]=1,otherwise,u[n]=0.In the aforementioned equations, R[n] and R[n] and R[n−1] are transmitted from the transmitter, ‘a’ and ‘b’ represent the amplitude of the transmitted signal, and u[n] is the amplitude bit determined by the receiver. In practice, if |R[n−1]| on the right side of the inequality is moved to the left side, and then it is equivalent to a division.
The fourth conventional demodulation technique is disclosed in U.S. Pat. No. 6,046,629 titled “Method of an apparatus for demodulating amplitude-modulated signals in an amplitude-phase-shift-keying (APSK) system”. The fourth conventional demodulation technique can be regarded as using the third conventional demodulation technique in a corresponding demodulation device. The threshold value acquisition circuit and the amplitude change decision circuit are major devices.
In the D-APSK demodulation technique, the D-PSK signal demodulation is simple while the D-ASK signal demodulation is sophisticated. The D-ASK signal demodulation is based on the concept of using divider. However, division circuits are more sophisticated. Moreover, demodulation based on division frequently leads to enhance noise effect in the demodulated signals and reduces the demodulation capacity. Hence, how to effectively demodulating D-ASK signals has become a critical point in designing the receiver of a communication system.